The present invention provides a slab type gas laser for generating an excellent output laser beam having substantially Gaussian intensity distribution when it is focused by the lens. For this end, a pair of cylindrical electrodes 11, 12 of different diameter are disposed concentrically by way of spacers 13 and laser medium is filled in the gap between the two cylindrical electrodes 11, 12 to define a straight slab 1. Disposed at one end of the straight slab 1 is a ring-shaped trick mirror M1. Also disposed at the center of the one end of the straight slab 1 is an output mirror M2 to pass a part of the light and to reflect a part of the remaining light. On the other hand, disposed at the other end of the straight slab 1 is a w-axicon mirror M3. The relationship between the center offset Xm and the center position X0 of the trick mirror is set to Xm1.1 X0.